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CVE-2026-5740— Unauthenticated WebSocket binary frame causes denial of service in Mattermost Server

Quick assessment

Affected
Mattermost Mattermost
Exploitation
No confirmed in-the-wild exploitation; assess based on exposure
Recommended action
Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

Mattermost是美国Mattermost公司的一个开源协作平台。 Mattermost 11.6.0及之前的11.6.x版本、11.5.3及之前的11.5.x版本、11.4.4及之前的11.4.x版本和10.11.14及之前的10.11.x版本存在安全漏洞，该漏洞源于未正确验证msgpack编码的WebSocket帧，可能导致未经身份验证的远程攻击者通过特制二进制WebSocket消息崩溃服务器进程，造成完全服务中断。

CVSS 7.5 · High EPSS 0.34% · P27

Possible ATT&CK Techniques 1 AI

T1499 · Endpoint Denial of Service

Affected Version Matrix 9

VendorProduct Version RangeStatus
Mattermost Mattermost 11.6.0≤ 11.6.0 affected
11.5.0≤ 11.5.3 affected
11.4.0≤ 11.4.4 affected
10.11.0≤ 10.11.14 affected
11.7.0 unaffected
11.6.1 unaffected
11.5.4 unaffected
11.4.5 unaffected
… +1 more rows
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I. Basic Information for CVE-2026-5740

Vulnerability Information
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Vulnerability Title
Unauthenticated WebSocket binary frame causes denial of service in Mattermost Server
Source: CVE Program / CVE List V5
Vulnerability Description
Mattermost versions 11.6.x <= 11.6.0, 11.5.x <= 11.5.3, 11.4.x <= 11.4.4, 10.11.x <= 10.11.14 fail to properly validate msgpack-encoded WebSocket frames before memory allocation which allows an unauthenticated remote attacker to crash the server process and cause a full service outage for all users via a crafted binary WebSocket message sent to the public WebSocket endpoint.. Mattermost Advisory ID: MMSA-2026-00647
Source: CVE Program / CVE List V5
CVSS Information
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Source: CVE Program / CVE List V5
Vulnerability Type
未经控制的内存分配
Source: CVE Program / CVE List V5
Vulnerability Title
Mattermost 安全漏洞
Source: CNNVD (China National Vulnerability Database)
Vulnerability Description
Mattermost是美国Mattermost公司的一个开源协作平台。 Mattermost 11.6.0及之前的11.6.x版本、11.5.3及之前的11.5.x版本、11.4.4及之前的11.4.x版本和10.11.14及之前的10.11.x版本存在安全漏洞，该漏洞源于未正确验证msgpack编码的WebSocket帧，可能导致未经身份验证的远程攻击者通过特制二进制WebSocket消息崩溃服务器进程，造成完全服务中断。
Source: CNNVD (China National Vulnerability Database)
CVSS Information
N/A
Source: CNNVD (China National Vulnerability Database)
Vulnerability Type
N/A
Source: CNNVD (China National Vulnerability Database)

Affected Products

Vendor Product Affected Versions CPE Subscribe
Mattermost Mattermost 11.6.0 ~ 11.6.0 -

II. Public POCs for CVE-2026-5740

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III. Intelligence Information for CVE-2026-5740

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Same Patch Batch · Mattermost · 2026-05-22 · 8 CVEs total

CVE-2026-4635 6.5 MEDIUM Persistent notification timing attack causing server denial of service
CVE-2026-5755 6.5 MEDIUM Denial of service via crafted TIFF file upload
CVE-2026-3473 5.9 MEDIUM Improper file ownership validation in the Boards API allows unauthorised file access
CVE-2026-28735 5.4 MEDIUM GitHub OAuth Scope Validation
CVE-2026-5308 4.9 MEDIUM Missing request body size limits on Zoom plugin HTTP endpoints
CVE-2026-4646 4.3 MEDIUM Insufficient input validation in GitHub plugin API causes denial of service
CVE-2026-3636 4.3 MEDIUM Sanitize team member data returned by API

IV. Related Vulnerabilities

Same product: Mattermost
Same vendor: Mattermost
Same weakness: CWE-789

V. Comments for CVE-2026-5740

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